The present applicant has proposed various vehicle seats using a tension structure formed by extending a three-dimensional knit and the like on a back frame and a cushion frame as a cushion material. Among them, Patent Literature 1 discloses a structure in which, if a force is applied in a direction to push in the tension structure extended on the back frame by a rear collision, a side frame collapses inward and moreover, bends rearward so as to receive a human body and to suppress a rebound. Moreover, the back frame of Patent Literature 1 has a configuration in which a frame for a head rest whose upper end position somewhat protrudes forward and forms a forward tilting posture is integrally provided. As a result, the side frame of the back frame bends rearward and suppresses a rebound of the human body while rearward inclination of the head part can be suppressed, and thus, a high whiplash reduction effect can be obtained. According to this back frame, since the whiplash reduction effect is high, there is no need to provide an active head rest mechanism, which contributes to weight reduction of the seat.
Moreover, Patent Literature 2 discloses a technology in which a portion with increased strength by heat treatment is partially provided on a frame so that the frame is deformed rearward from the portion having relatively low strength not subjected to heat treatment when an impact larger than a predetermined level is applied.